Rats can hear sounds that we cannot: they can hear ultrasonic frequencies well above the range of human perception. Rats produce ultrasound, too, and communicate with each other in squeaks, clicks, and whines that we cannot hear.
Humans can hear sounds from about 16 to 20,000 Hz (20 kHz)*. Anything above 20 kHz is called "ultrasound," because those sounds are higher than we can hear. Anything below 20 Hz is called infrasound -- elephants communicate over miles with low, rumbling infrasound (more).
Rats can hear ultrasound: the range of the rat's hearing is around 200 Hz to 80 or 90 kHz (Fay 1988, Kelly and Masterson 1977, Warfield 1973). There is a whole world of high frequency sound out there that rats can hear that we cannot, a perceptual difference that humans tend to forget (Milligan et al. 1993, Sales et al. 1998).
For example, when a human gently rubs thumb and forfinger together, we hear nothing. But this movement makes a scratchy sound in the ultrasonic range. Wire cages make a lot of ultrasonic noise in addition to audible noise when rats move around in them.
Here are some of the frequency ranges that are perceptible to other species:
Further reading on sound:
* Quick refresher on the Hertz: A Hertz is a measurement of frequency in cycles per second. A thousand Hertz is a kilohertz, or kHz (1,000 Hz = 1 kHz). The higher the Hertz, the higher pitched the sound. Middle C has a frequency of 263 Hz. The lowest note on a piano is 27 Hz, and the highest note is 4186 Hz. The human voice ranges from about 100 to 1700 Hz, and cymbals can make a sound as high as 15,000 Hz. Humans can't hear much below 20 Hz. Below 20 Hz humans may sense the sounds as vibrations.
Mammals pinpoint the source of sound using several types of cues. The first two types of localization cues use the fact that we have two ears separated by the width of our heads: a sound coming from the left will reach the left ear slightly before the right ear, and the sound will be louder in the left ear than the right. The brain uses the time delay and difference in loudness to pinpoint the source of sound in space in the left-right plane.
The third type of cue involves the outer ear (pinna). Sounds coming from the front of the ear will be louder than those coming from the back. Also, sound is modified as it passes over the ear's many bumps and convolutions, and will therefore sound different depending on whether it comes from above or below. Therefore, the outer ear helps localize sound in space in the front-back and up-down planes (Hofman et al. 1998). [Note: This is why sound appears to surround you when you wear headphones: the headphones eliminate the sound localization cues provided by the outer ear (Oldfield and Parker 1984).]
Small animals have only a short distance between their ears, so they have less time delay to work with than large animals. Small animals, like rats, are therefore not as good at pinpointing sources of sound as large animals (this is called low sound localization acuity) (Kelly and Phillips 1991).
Rats can pinpoint the location of a sound to within about 12 degrees for clicks or 9.7 degrees for a burst of white noise (these are measures of chance level localization thresholds; Heffner and Heffner 1985). Similarly, Kavanagh and Kelly (1986) found a sound localization of 11.1 degrees in rats. Humans, in contrast, have greater sound localization acuity. We can pinpoint sounds in front of us to within 2 to 3.5 degrees (average localization error; Makous and Middlebrooks 1990).
In many species, such as cats (Conlee et al. 1984) and humans (Creel et al. 1980) albinism is associated with hearing impairments (also see Creel 1980, Deol 1970).
This is not the case for rats. Albino rats have impaired vision, and an impaired sense of smell, but albinos appear to have normal hearing. Sound localization acuity is similar for albino and pigmented rats (Heffner and Heffner 1985). Albinos can discriminate between sounds of different frequency (pitch) and intensity (loudness) just as well as pigmented rats (Syka et al. 1996), and have a normal hearing range (Kelly and Masterson 1977).
Therefore, albinism appears to have no effect on hearing in rats.
What ultrasounds can a rat make?
20 kHz range: Rats emit long 20 kHz vocalizations when they are unhappy or stressed. These calls are emitted when an adult or juvenile is defeated socially (Thomas 1983), sees a predator (Blanchard 1991), experiences pain (Cuomo 1988, Tonue 1986) or anticipation of pain (Antoniadis 1999), or when an untame rat is handled (Brudzynski and Ociepa 1992).
30 to 50 kHz range: Infant rats produce very high pitched distress calls. These cries elicit maternal care such as retrieving the infants to the nest (Allin and Banks 1971; Carden and Hofer 1992).
Infants may also call when their mother steps on them. Infants who can't call are stepped on more roughly by their mothers, so the calls of infants in this case may reduce rough handling by their mothers (Allin & Banks 1972, Hofer & Shair 1978, Noirot 1972, White 1992).
Rats also emit short, high-pitched calls under positive contexts. Adults and juveniles emit them during rough and tumble play (Knutson 1998, Burgdorf and Panksepp 2001, Panksepp and Burgdorf 1998), and in anticipation of feeding (Burgdorf 2000).
Male and female rats also call in a sexual context (Barfield 1979). Before copulation, males and females emit such calls as they approach and sniff each other. When males emit these calls, females solicit them more (McIntosh et al. 1978). Female calling also solicits male sexual behavior. Calling may also coordinate the sequence of behaviors that leads to intromission (White 1998).
Further reading on rat ultrasound:
Rats aren't the only animals that can make high pitched sounds.
Bats are the most famous emitters of ultrasound, with their inaudible echolocation clicks in the 25 to 80 kHz range. But bats aren't the only ones, shrews produce 20 to 64 kHz calls during exploration. And dolphins, who also use echolocation to navigate and locate prey, have trumped the bats: their echolocation clicks range from 80 kHz to an incredible 150 kHz.
Insects produce high pitched sounds as well.Some moths court by fanning their wings at 80 kHz. Some ant species produce pulses at 75 kHz by rubbing parts of their hard exoskeleton together (Sales 1974).
Bats and dolphins use ultrasound to navigate and hunt. High frequency sound waves are so tiny that they penetrate into and reflect off of the smallest crevice and contour, echoing back a sound profile in minute detail.
Humans produce ultrasonic sounds as well. Ultrasonic sound waves can clean surfaces much better than a solvent, by vibrating and dislodging tiny particles of dirt. Hospital medical cleaners range from 25,000 to 38,000 Hz, and jewelers use ultrasonic cleaners to clean jewelry, at 42,000 kHz.
We use ultrasound much like a dophin does when we view fetuses in the womb. Ultrasound machines emit extremely high frequencies, from 1,000 kHz to 20,000 kHz. The machine reads the echoes that bounce off the fetus and translates them into a visual image on a screen.
Ultrasonic pest repellers are quite popular. The manufacturers claim that their devices produce ultrasonic noise so aversive to pests (including rats) that it drives the pests away. These devices are appealing to consumers because they are silent to human ears and don't involve traps or poison.
Ultrasonic collars are supposed to drive fleas off dogs and cats. Ultrasonic devices are supposed to chase away rodent, bird, and insect pests. Ultrasonic emitters on cars are supposed warn large animals away from roads. Lastly, manufacturers frequntly claim that these devices are safe for pets. Are these claims true?
Do ultrasonic pest repellers drive away pests?
No. Controlled studies have shown that ultrasonic pest repellers are ineffective at driving away wild rats and other pests. Specifically, ultrasound is no more effective than audible sound (Bomford and O'Brien 1990).
Ultrasonic collars do not drive fleas off of cats and dogs. Ultrasound does not drive away fleas or cause any change in flea activity patterns (Brown and Lewis 1991, Koehler et al. 1989). Ultrasonic cat collars have no effect on flea egg-laying, development of larvae, or flea mortality (Hinkel et al. 1990). Ultrasonic collars made no difference to the number of fleas on cats (Dryden et al. 1989).
Ultrasound does not keep wild animals away from roads. Ultrasonic devices did nothing to alter the behavior of moose (Muzzi and Bisset 1990), mule deer (Romin and Dalton 1992), or kangaroos (Bender 2001).
Ultrasonic devices are ineffective at repelling pests. Ultrasonic pest repellers do not repel cockroaches (Gold et al. 1984, Ballard et al. 1984), mosquitoes (Sylla and Kremsner 2000, Foster and Lutes 1985), white-tailed deer (Belant et al. 1998, Curtis et al. 1997), bats (Hurley and Fenton 1980), cats (Mills et al. 2000), starlings (Bomford 1990), pigeons (Griffiths 1988, Woronecki 1988), and many other bird species (Hamershock 1992).
Ultrasonic pest repellers do little to repel unwanted rodents. Rodents may be driven away for a few minutes or a few days, but they tend to return to their nesting and feeding areas even in the presence of ultrasound (Pierce 1993, Timm 1994). Ultrasound has not been shown to drive rodents out of buildings or to cause above-normal levels of mortality (Timm 1994).
Why don't ultrasonic pest repellers work?
Ultrasonic devices do not work because rodents rapidly become accustomed to repeated sounds (a process called habituation). Mice and rats learn that the ultrasound from the pest repeller isn't dangerous, so they gradually get used to it and return to their nesting and feeding areas.
In addition, ultrasound is quite weak and attenuates very rapidly with distance from the source (Lawrence and Simmons 1982). Half the energy of ultrasound produced by pest repellers is gone at 15 feet, and no energy remains at 30 feet. Ultrasound is therefore very short-range (Askham 1992).
Lastly, ultrasound is blocked by intervening objects like walls and furniture. Ultrasound cannot travel through these objects and cannot go around corners. Therefore, walls, doors, and furniture cast "sound shadows" behind them. Rats and mice can easily use these silent areas for auditory shelter, thus avoiding the ultrasound altogether.
Figure 1. Diagram of sound shadows and attenuation of ultrasound in a living room. The ultrasonic pest repeller sits on the table. It emits sound (blue). The sound is blocked by furniture and walls, creating"sound shadows" which mice can hide in -- behind the couch, inside a wall, under the table the device sits on. Ultrasound attenutates rapidly with distance from the device: half the energy produced by the pest repeller is gone at 15 feet, and no energy remains at 30 feet.
Note: It is possible to cause convulsions and permanent damage with ultrasound, but the intensity (loudness) of such sounds must be so great that they would damage humans and domestic animals as well. Commercial ultrasonic pest control devices do not produce sounds of such intensity (Timm 1994).
Do ultrasonic pest repellers harm pets?
Only a few tests have been done on the effect of ultrasonic pest repellers on pets. Dogs displayed no aversive affects to ultrasound (Blackshaw et al. 1990). Cats do not become alert or take flight when they hear ultrasound. Cats tended to explore less and actually stayed closer to the device when it was on (Mills et al. 2000). These preliminary studies indicate that ultrasonic pest repellers do not have an immediate, short term averse affect on pets. No studies have yet examined the effect of prolonged exposure to ultrasonic pest repellers on the health of any pets.
Further reading on ultrasonic devices: